At your serving temp 15 psi is about the correct carb level. I think you are normally looking at 10-14 days to get up to your full carb level if you are set to the correct psi.
There are a couple ways to cut that time down some. The safest way to quick carb is to set your regulator about 10-15 psi over your ideal setting for a day or two. So if you set yours to say 25 psi, and then check the carb level after 24 hours, and then every 12 hours after that to make sure you don't push it to far (over carb). Usually 24-36 hours gets you to your carb level, but since you've been on gas for 4 days already you might get there a bit quicker. Then just turn it back down to 15psi when it gets where you want it.
Of course the most safe way is the slow and steady like you are doing, but I usually bump mine up to get it where I want it in a couple days.